Elastic rail clamps of this kind are described in U.S. Pat. Nos. 3,067,947 (Deenik) and 4,313,563 (Young). Generally both these clamps are designed for use with a custom made shoulder into which the base of the elastic clamp fits. Both rail clamps have toe portions which lie on the rail flange and the outer edges of these abut the support shoulder to prevent them sliding off the rail flange. The "Young" clip is used with a shoulder as described in U.S. Pat. No. 4,688,719 which has a shoulder with a tapered slot leading to the gate which acts to compress the arms together as the clamp is driven into place with the toe portion of the rail flange.
However, many railway tracks that are already laid have existing rail plates or shoulders but could be upgraded at low cost by utilizing an elastic rail clamp without having a custom built support shoulder. A common rail plate used in Europe is called a K plate which includes a pair of parallel ribs adjacent and parallel to the rail flange and slots in the ribs at right angles to the rail flange. These slots are not tapered and thus the toes of the rail clamps need to be compressed to pass through the slot if the toes are to be prevented from sliding off the rail flange. The Deenik clip can only be applied to the K plate with a tool which compresses the arms of the toe portion to pass through the gate in the shoulder. With older rail tracks the variation in height between rail plate slot and rail flange can vary greatly in stationary and dynamic modes and thus maximum clamp deflection between the base of the toe portions is needed.